The invention is applied to scintillation materials and may be used in nuclear physics, medicine, and oil industry for recording and measuring of X-ray, gamma- and alpha-radiation; non-destructive testing of solid state structure; three-dimensional positron-electron computer tomography (PET) and X-ray computer fluorography. The relevance of the invention is that in fluoroscopy, X-ray computer tomography and PET, an introduction of new/improved scintillators has resulted in significant improvement of the image quality or/and reduced the measuring time. (“Inorganic scintillators in medical imaging detectors” Carel W. E. van Eijk, Nuclear Instruments and Methods in Physics Research A 509 (2003) 17–25).
The known scintillation substance is a lutetium oxyorthosilicate powder doped with cerium Lu1.98Ce0.02SiO5 (A. G. Gomes, A. Bril “Preparation and Cathodoluminescence of Ce3+ activated yttrium silicates and some isostructural compounds”, Mat. Res. Bull. Vol. 4, 1969, pp. 643–650). This phosphor was created for an application in the cathodoluminescence devices, however this substance may be utilized also for the X-ray, gamma- and alpha-ray emissions recording.
It is known the scintillation substance/crystal of cerium doped lutetium oxyorthosilicate Ce2xLu2(1−x)SiO5, where x is varied between the limits from 2×10−4 to 3×10−2 (U.S. Pat. No. 4,958,080, Sep. 18, 1990). The crystals of this composition are grown from a melt having composition of Ce2xLu2(1−x)SiO5. In scientific literature abbreviated name LSO:Ce is wide used for denotation of this crystal. The Ce2xLu2(1−x)SiO5 scintillation crystals have a number of advantages in comparison with other crystals: a high density, a high atomic number, relatively low refractive index, a high light yield, a short decay time of scintillation. The disadvantage of known scintillation material is the large spread of important characteristics of scintillation, namely, a light yield and an energy resolution, from crystal to crystal. The experimental results of systematic measurements of commercially produced LSO:Ce crystals grown by CTI Inc. company (Knoxville, USA) clearly display this (U.S. Pat. No. 6,413,311, Jul. 2, 2002). Another disadvantage is a significant reduction of light yield, when the containing LSO:Ce crystal device is operated under conditions when the temperature is above a room temperature, for example, in petroleum industry for the rock composition analyses in a borehole during the search of the new deposits. Another disadvantage of LSO:Ce crystals is an afterglow effect, that is the prolonging fluorescence after radiation exposure, for example, the luminescence intensity of the samples described in U.S. Pat. No. 4,958,080 is reduced to decibels during ten minutes.
It is known the scintillation substance the lutetium oxyorthosilicate containing cerium, Ce:Lu2SiO5, in the form of a transparent ceramics. The Lu2SiO5:Ce scintillator is formed into ceramics material through sintering the Lu2SiO5:Ce powder. Because the Lu2SiO5:Ce has a monoclinic structure rather than a cubic crystalline structure, the sintering produces a translucent ceramics rather than transparent. The cerium-doped lutetium orthosilicate is formed into a transparent glass scintillator by combining the silicate oxide, lutetium oxide, cerium oxide, potassium oxide, and barium oxide. The pores between the particles are removed which results in a consolidation of the scintillator material. As a result, the translucent ceramics is converted into a transparent ceramics applicable for using in the medicine tomographs (U.S. Pat. No. 6,498,828 from Dec. 12, 2002). The drawback of patent proposed is a quality of scintillation ceramics, which is made from, so-named, stoichiometric composition of lutetium oxyorthosi-licate mixture, a stoichiometric composition is characterised by ratio of formula units of (Lu+Ce)/Si is equal exactly to 2/1. Since the congruent composition of lutetium oxyorthosilicate does not coincide with stoichiometric one, the ceramics of stoichiometric composition apparently contains the components of oxides which did not react completely as a results the scattering centers are formed. The light yield is an important characteristic of a scintillator. The presence of scattering centers reduces a light yield appreci-ably. A transparent ceramics made from a cerium-doped gadolinium oxyorthosilicate has the same limi-tation (W. Rossner, R. Breu “Luminescence properties cerium-doped gadolinium oxyorthosilicate ceramics scintillators” Proc. Int. Conf. on Inorganic Scintillators and Their Application, STINT'95, Netherlands, Delft University, 1996, p. 376–379). The scintillation elements fabricated from the transparent ceramics have the 60% less light yield than the elements fabricated from the Ce:Gd2SiO5 crystals.
Presence of an afterglow is very unwanted effect for some applications, for example, for an imaging system, in which the electronic part of device indicates a photon flux from the scintillation elements absorbing the gamma radiation. The afterglow effect, i.e. a photon flux from the scintillation element does not exposed to gamma radiation, reduces a contrast range, a sensitivity and a precision of device. The afterglow impairs also the parameters of medical devices based on the utilization of positron emitting isotopes, for example, the three-dimensional medical tomographs (Fully-3D PET camera) for diagnostic of the cancer diseases, and, especially, for the MicroPET systems designed for testing of the new medicines. A principle of operation of the three-dimensional medical tomographs is that the microscopic concentration of substance containing an emitting positron isotope is introduced into the blood of a patient. This substance is accumulated in the cancer cells of patient. An emitted positron annihilates instantly with an electron this results in the emission of the two 511 KeV energy gamma-quantums scattering exactly in opposite directions. In tomograph the detection of both gamma-quantums occurs by means of the several ring detectors each of which contains hundreds of the separate crystalline scintillation elements. The high Ce:LSO density gives an effective absorption of all gamma quantums emitting from a body of patient examined. A location of the atom of a radioactive isotope in a patient body is determined by means of a time detection of both gammas and numbers of scintillation elements indicated these gamma quantums. In a patient body a part of gamma quantums is scattered because of Compton effect, as a result, the detection of gamma quantums occurs by the crystalline scintillation elements do not arranged in line. Therefore if an scintillation element has a strong afterglow then the indicating system may recognise it as a result of annihilation at a moment, however, actually, this detection is a consequence of exposure to gamma quantum radiation in previous moment of measuring. In the three-dimensional medical tomographs of regular resolution the several thousands 6×6×30 mm3 scintillation elements are used, they maintain the 6×6×6=216 mm3 volume three-dimensional resolution. Even a strong afterglow of the Ce:LSO crystals does not lead up to the considerable consequences when the comparatively thick 6×6 mm2 cross-section elements are used for a diagnostics of the cancer illnesses, because a desired recording accuracy may be achieved by an injection of the large doses of radioactive substances or by a reducing of the rate of translation of patient through tomograph's ring.
However condition is changed sharply for MicroPET, which are used for a study of the life processes in vivo, especially, in a human brain or for a measuring of a distribution of medicines in a animal body (mouse, rats) during testing of the new medicines. For MicroPET systems it is necessary to use the devices with a maximal space resolution. The 1×1 mm2 sectioned and even 0.8×0.8 mm2 sectioned scintillation elements are used just now. The 1 mm3 space resolution is achieved. Because of so small thickness of elements the numerous gamma quantums may cross direct the several scintillation elements at different angles. Consequently, to calculate which part of a scintillation radiation is induced by some or other gamma quantum is a complicate technical task. In this case an afterglow becomes a very undesirable effect, because it reduces an accuracy all system.
The afterglow and thermoluminescence phenomena are explored circumstantially for the Ce:LSO crystals (P. Dorenbost, C. van Eijekt, A. Bost, Melcher “Afterglow and thermoluminescence properties of Lu2SiO5:Ce scintillation crystals”, J.Phys.Condens.Matter 6 (1994), pp. 4167–4180). According to this article an afterglow is observed both in the crystals having a high light yield and a low light yield, and a conclusion is that an afterglow is a property immanent to the Ce:LSO substance.
It is known substance the cerium doped gadolinium oxyorthosilicate, Ce2yGd2(1−x−y)A2xSiO5, where A is at least one element selected from the group La (lanthanum) and Y (yttrium), the x and y values are varied within the limits 0<x<0.5 and 1×10−3<y<0.1 (U.S. Pat. No. 4,647,781, Mar. 3, 1987). The main limitation of this group of scintillation crystals is a low light yield in comparison with the Ce-doped lutetium oxyorthosilicate, Ce2xLu2(1−x)SiO5, described above.
The known method of crystal growing of the large size Ce-doped lutetium oxyorthosilicate, Ce:LSO, is described in the U.S. Pat. No. 6,413,311, where the Ce:LSO boules up to 60 mm in diameter and 20 cm long are grown by Czochralski technique. An appreciable demerit of these large-sized Ce:LSO boules is that a light yield is strongly differed even within a boule, decreasing to 30%–40% from a top to a bottom of a boule. Furthermore, a scintillation decay time (a time of luminescence) may be varied over the wide range of values from 29 nanoseconds to 46 nanoseconds, at that an energy resolution value may fluctuate within the 12%–20% limit. Such a large spread in performance leads up to necessity during an industrial production to grow a large number of boules by Czochralski method, to cut them into parts (packs), to test each pack and on the basis of such tests to select the packs which possibly to utilize for fabrication of scintillation elements for medical tomographs.
It is known the scintillation crystals, LU2(1−x)Me2xSi2O7, where LU is lutetium-based alloy which also includes one or more of Sc, Yb, In, La, and Gd; where Me is Ce or cerium partially substituted with one or more of the elements of the lanthanide family excluding lutetium; and where x is defined by the limiting level of LU substitution for Me in a monoclinic crystal of the lutetium pyrosilicate structure (U.S. Pat. No. 6,437,336). The crystal is formed by crystallization from a congruent molten composition of Lu2(1−x)M2xSi2O7, a congruent composition allows to use up to 80% of initial melt, and the crystals exhibit reproducible scintillation response to gamma radiation, a light yield spread over volume of boule did not exceed 20% and this commercial parameter was significantly better than for Ce:LSO crystals. However, the Lu2(1−x) Me2xSi2O7 crystals appreciably conceded to the Lu2SiO5 crystals in the basic scintillation parameters, namely, the light yield and density. Thus the lutetium oxyorthosilicate crystals, Ce:LSO, are a more preferable scintillator for utilization in a three-dimensional positron-electron tomography, because a tomograph based on these crystals is a more sensitive and, in consequence, a dose of radioactive medicaments, adding in the blood of a patience on early stage of cancers, is reduced.
It is known the lithium containing scintillation substance of the cerium doped yttrium silicate of chemical formula LiHSiO4, (M. E. Globus, B. V. Grinev “Inorganic scintillators”, publishing house ‘AKTA’ Kharkov, (2000) p. 51). The 5%Ce3+-doped LiYSiO4 crystal has a peak of luminescence at 410 nm, a luminescence time constant is equaled to 38 ns and a maximal light yield at detection of gamma quantums is 1000 photons/Mev, this value is two and half time less than for the known lutetium oxyorthosilicate scintillating crystals, Ce2xLu2(1−x)SiO5. A low efficient detection of gamma radiation is resulted from a low density of scintillator is equaled 3.8 g/cm3. This substance may be utilized for detection of neutron radiation, however material is a low efficient for a gamma radiation.
It is known the lithium containing scintillation substance of the cerium doped lutetium silicate of chemical formula LiLuSiO4, (M. E. Globus, B. V. Grinev “Inorganic scintillators”, publishing house ‘AKTA’ Kharkov, (2000) p. 51). The 1%Ce3+-doped LiLuSiO4 crystal has a peak of luminescence at 420 nm, a luminescence time constant is equaled to 42 ns and a maximal light yield at detection of gamma radiation is about 30000 photons/Mev, this value is 10% higher than for the known lutetium oxyorthosilicate scintillating crystals, Ce2xLu2(1−x)SiO5. However, an essential limitation of given crystal is a low density equaled to 5.5 g/cm3. Such small density does not allow to use these crystals in tree-dimensional tomographs (Fully-3D PET camera) and, especially, for MicroPET systems, because the basic requirement for scintillating crystal for these applications is an attenuation length of gamma radiation, which should be less then 1.5 cm (W. M. Moses, S. E. Derenzo “Scintillators for positron emission tomography”, Conference SCINT'95, Delft, The Netherlands (1995), LBL-37720). This parameter is equaled 2.67 cm for crystal having a density of 5.5 g/cm3, whereas for the Ce2xLu2(1−x)SiO5 crystal of 7.4 g/cm3 density an attenuation length is equaled 1.14 cm.
The Ce:LiYSiO4 and Ce:LiLuSiO4 crystals can not be recognised as a prototype for any variants of the given invention, because they are differed both a chemical formula and a crystal structure, which defines a crystal density. A high crystal density is a basic parameter for the applications which are the aim of the given invention.
The chemical formulae of the given invention are the numerous crystals of the solid solutions on the basis of the silicate crystal containing a cerium, Ce, and crystallising in the monoclinic syngony, spatial group B2/b, Z=4, and crystallising in a hexagonal syngony of apatite structural type with a spatial group P63/m, Z=1.
It is known the mono-cation cerium silicate crystallising in an apatite-brytolite structural type, Ce9.33□0.67(SiO4)6O2, where □ is a cation vacancy (A. M. Korovkin, T. I. Merkulyaeva, L. G. Morozova, I. A. Pechanskaya, M. V. Petrov, I. R. Savinova “Optical and spectral-luminescence properties of the orthosilicate crystals of lanthanide” Optics and Spectroscopy, value 58, issue 6 (1985) p. 1266–1269) and the double silicate of cerium, LiCe9(SiO4)6O2, (I. A. Bondar, N. V. Vinogradova, L. N. Dem'yanets et al. “Silicates, germanates, phosphates, arsenates, and vanadates. Chemistry of rare elements” monograph M. Nauka, (1983) 288 p.). A cerium presents in the Ce9.33□0.67Si6O26 and LiCe9Si6O26 crystals, however, a luminescence is completely quenched in them, this is explained by a concentration quenching in consequence of high concentration of cerium ions in crystals. These crystals are not applicable for utilization as a scintillator. An analogue of the substance claimed in the items 16, 17, 18 of given invention is a crystal of mono-cation cerium silicate, Ce9.33□0.67Si6O26, since it has the same symmetry, P63/m, Z=1, and has a closest composition to the variants aforecited. An analogue of the substance claimed in the items 19, 20, 21 of given invention is a crystal of double cerium silicate, LiCe9Si6O26, since it has the same symmetry, P63/m, Z=1, and has a closest composition to the variants aforecited. Both the Ce9.33□0.67Si6O26 crystal and the LiCe9Si6O26 crystal cannot be accepted as prototypes for each variant of scintillation substance of given invention since they are not a scintillation material, i.e. these crystals do not have a generic character of given invention reflecting a purpose.
A computer search of chemical compounds in the international X-ray library's database (PDF Database, International Center for Diffraction Data, Newton Square, Pa., U.S.A.) has shown that the individual chemical compounds on a basis mono-cations and doubles silicates, R9.33□0.67(SiO4)6O26 and LiR9Si6O26, respectively, where R=La, Sm, Nd, Gd, Ce are known. However, to our knowledge, there are no patents or publications in which these compounds were additionally doped with cerium what is necessary for an initiation of scintillation properties. Therefore the R9.33□0.67(SiO4)6O2 and LiR9Si6O26 substances, where R=La, Gd or their mixture, it is necessary to consider as an utilization of known substance on a new purpose.
The nearest analogue chosen as a prototype for all variants of the claimed scintillation substance, is a scintillation substance (variants) patented in the 2157552 patent, Russia, and the U.S. Pat. No. 6,278,832 patent, USA. The chemical formulae of this invention represent the numerous crystals of solid solutions of oxyorthosilicate crystal, including cerium, Ce, and crystallising in the Lu2SiO5 structural type with space group B2/b, Z=4, which composition is represented by the chemical formula CexLu1A1−xSiO5, where A is Lu and at least one element selected from the group consisting of Gd, Sc, Y, La, Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, and Yb. Other elements of periodic table can be occurred in a crystal as the impurities in the starting oxides or can be introduced into composition during a crystal growth or in a result of annealing in a special atmosphere. Partially the similar results are achieved in the U.S. Pat. No. 6,323,489. This patent protects the lutetium-yttrium oxyorthosilicate crystal of composition having the chemical formula CezLu2−x−zYxSiO5, where 0.05<x<1.95 and 0.001<z<0.02. The main disadvantage of the above mentioned inventions is the use only molar ratio equaled to 50%Lu2O3/50%SiO2=1 of starting oxides for all patented scintillation materials, that corresponds exactly to stoichiometric composition of Lu2SiO5 structure. For all mixed crystals simultaneously containing several rare-earth ions, the ratio of 50% of the mix of different elements and 50% of SiO2 has been used. This composition does not allow to grow by Czochralski method the large commercial (diameter more than 80–100 mm) coating lutetium and Ce-doped crystals having a high uniformity of scintillation parameters on all volume of boule. Additionally, the crystals of stoichiometric composition cracked when being sawed for scintillation elements, for example, in the size of 0.8×0.8×10 mm3. Another essential disadvantage of specified scintillation materials is the presence of oxygen vacancies which increase a light output and reduce a probability of cracking of the boules at sawing, however, simultaneously, the presence of oxygen vacancies in two–four times increases an intensity of afterglow (thermoluminescence) after gamma-radiation of scintillation material.
Another confirmation of basic drawback of composition characterised by the 50%/Lu2O3/50%SiO2 molar ratio of oxides is the information described in U.S. Pat. No. 5,660,627. This patent protects a method of growing of lutetium orthosilicate crystal with a plane front of crystallization by Czochralski method from a melt of Ce2xLu2(1−x)SiO5 chemical formula, where 2×10−4<×<6×10−2. The gamma luminescence spectra of crystals grown with a conical front of crystallization and with a plane front of crystallization have the strong, fundamental differences both in a shape and in a position of maximum of luminescence. So the appreciable differences result from the composition of the initial melt, which has the 50%Lu2O3/50%SiO2 mole ratio of main components. A crystal growing from this melt has a composition differed from the composition of melt, the gradient of concentration is observed along a crystal cross-section, and the real Ce2xLu2(1−x)/Si ions ratio is differed from the ratio of 2/1=2 formula units. For the confirmation of the main declared in the U.S. Pat. No. 5,660,627 the crystals 26 mm in diameter were grown at the 0.5 mm/hour and 1 mm/hour rates, however, even at these very advantageous growth parameters, the crystals grown with a conical crystallization front can not be used for the commercial applications because of cracking and spread of scintillation performance.
For many years the growing of crystals with a planar crystal-melt interface by Czochralski method is used for commercial production of optical and piezoelectric materials, that is described in detail in the hundreds of papers in scientific journals and books. The well known commercial lithium metaniobate crystal (R. L. Byer, J. F. Young “Growth of High-Quality LiNbO3 Crystals from the Congruent Melt” Journal of Appl. Phys. 41, N6, (1970), p. 2320–2325) is being grown by Czochralski method from a melt of congruent composition, Li0.946NbO2.973. having the ratio of initial oxides is equaled to Li2O/Nb2O5=0.946, the congruent composition is differed from an ordinary, stoichiometric composition of lithium metaniobate, LiNbO3, where a ratio of component is equaled to 50%LiO/50%Nb2O5=1. (P. Lerner, C. Legras, J. Dumas “Stoichiometrie des mohocristaux de metaniobate de lithium”, Journal of Crystal Growth, 3,4 (1968) p. 231–235). An existence of non-stoichiometric compounds is directly concerned with a structure of real crystal, in which the vacant lattice sites exist, and the excess atoms of one of the elements are placed in the crystal interstitial sites. (P. V. Geld, F. A. Sidorenko “Dependence of physical-chemical properties of non-stoichiometric compounds on structure of short-range order” Izvestia AN SSSR, seria Inorganic materials, 1979, v. 15, #6, p. 1042–1048). As a result, a ratio of components forming a structure does not correspond to the whole-numbered indices, and the chemical formulae of such compounds are described by the fractional numbers. A chemical composition is named the congruent composition, if a composition of melt is coincided with a composition of crystal growing from this melt. All the physical and mechanical properties of crystals grown from the melts of congruent compositions maintain the values constant over all volume of boule. For some applications a near stoichiometric composition, Li2O/Nb2O5=1, is a preferable use, U.S. Pat. No. 6,464,777 B2 dated Oct. 15, 2002. This patent clearly illustrates as the small variations of crystal composition lead up to the appreciable alterations of physical properties of crystal and this is important for the practical applications.
It is known (in the book D. T. J. Hurle “Crystal Pulling from the Melt” Springer-Verlag, Berlin, Heidelberg, New-York, London, Paris, Tokyo, Hong Kong, Budapest, 1993, p. 21) that because of the complex oxide systems of optics and electronics interests, such as garnets and spinels, do not correspond to a congruently melting composition it is necessary to induce growth only at a very low rate in order to give time for diffusion away from the interface of the excess component. Failure to do this leads to dramatic degradation in the perfection of the crystals due to the occurrence of constitutional supercooling. A search of congruent composition or very near to congruent composition is an important stage of development of commercial production of all optical materials, however, the authors of given invention do not know the data about congruent composition (or near to congruent composition) of lutetium oxyorthosilicate published in the scientific journals or in the patents. All known publications are dedicated to the crystals, in which a ratio of formula units, (Ce2x+Lu2(1−x)/Si, is exactly equaled to 2/1.
Generalising the above-mentioned, we may conclude that a basic technical drawback, immanent to both the known scintillation crystals on the basis of lutetium orthosilicate, CexLu2xSiO5, and prototype's crystals and a method of making of these crystals, are a longitudinal heterogeneity of optical quality of grown crystals, a heterogeneity of the basic scintillation parameters both in a bulk of boule grown by Czochralski method and heterogeneity from boule to boule grown in alike conditions and, at last, a low growth rate. These drawbacks substantially arise from the use in Czochralski method of melt having a composition which characterised a ratio of formula units, (Ce+Lu)/Si, which is exactly equaled to 2/1, i.e. the reason of these drawbacks resides in a non-congruent composition of melt. At the existence of congruent point, a crystal growth from a stoichiometric composition leads up to that the segregation coefficients of both the host crystal components, Lu, Si, and the additional component, Ce, are differed from unit, and, moreover, a crystal composition is shifting from the congruent point as a crystal pulling, that results in dramatic degradation of crystal quality despite on the extremely low growth speed. A segregation coefficient of component is a ratio of component's quantity in a crystal to component's quantity in a melt. Another common technical demerit of scintillation crystals on the base of lutetium orhtosilicate is the large losses of crystalline material because of cracking during slicing of a large, up to 60 mm in diameter, boules into 1 mm thickness pieces, which in their turn are cut into rods to produce the 1×1×10 mm3 dimensions elements in the quantity of several tens of thousands pieces needed for assembling of one tomograph.